We’ve been receiving a lot of questions about the new Limited Clone class, and many of those questions revolve around carburetors. The primary reason is that some builders—though not a majority—are pushing for rule changes to allow 1.060” bore carburetors.
Before addressing that issue, it’s important to understand why the original rules specified a 1-inch bore Tillotson carburetor.
- The Briggs Animal Limited class served as the reference point for the 208cc Limited Clone class, utilizing 1-inch bore / .900” venturi carburetors. Because the clone platform is a more affordable option, aligning the carburetor specifications makes it easier for racers to transition from the Animal to the Clone. It also simplifies technical inspection, allowing tracks to use the same measuring tools and procedures.
- Carburetors such as the HL-334WX and HL-304WX (including G and A variants) were originally designed for OHV engines. These carburetors feature a 1-inch bore with a .880” or smaller venturi, which makes blueprinting engines to meet class rules more straightforward and consistent.
- The 1-inch bore carburetor has long been the traditional standard in Limited Modified classes. The 1.060” bore carburetors were introduced much later, near the end of Briggs’ support for flathead platforms, and were never part of the core foundation of the class.
- The 1-inch configuration is also the most widely produced size in the Tillotson HL-series, with over 60 years of production history. While not every carburetor is competitive out of the box, this long production run supports the argument that the 1-inch bore is the most accessible and widely available option
4000 RPM / 6000 RPM / 8000 RPM
- Briggs Flathead (2.437”)
- 1,625 ft/min / 2,437 ft/min / 3,249 ft/min
- Briggs Animal (2.204”)
- 1,469 ft/min / 2,204 ft/min / 2,939 ft/min
- 208cc Clone (2.126”)
- 1,417 ft/min / 2,126 ft/min / 2,835 ft/min
Unlike previous Limited 4-cycle rules, we are introducing a new billet diaphragm carburetor—the XR-25 Intimidator—to replace the traditional cast HL-304WX Tillotson carburetors.
The reasoning behind this shift is simple: the Tillotson HL-series has effectively been phased out. Once Briggs discontinued support for flathead platforms, classes such as Super Stock, Limited Modified, and Open Modified—many of which relied on Tillotson carburetors—gradually disappeared without being replaced by OHV equivalents. As a result, the market for these carburetors declined significantly. While Tillotson has introduced new models for modern 2-cycle engines, those carburetors typically retail in the $300–$500 range in stock form.
Because the original 4-cycle carburetor platform was largely developed by EC Birt, adapting newer carburetors would require entirely new fuel calibrations, revised linkages (and potentially new throttle shafts), and new intake manifolds. This represents a significant amount of additional development work just to offer a suitable “stock” carburetor. The cost of a complete package—including intake, carburetor, and linkage—would increase by approximately $100, with blueprinting adding even more. This directly conflicts with the affordability goals of the class.
Over the past 20 years, we have developed both cast and billet carburetors to replace obsolete 4-cycle platforms. To address the lack of new 1-inch OHV-compatible carburetors, we engineered the XR-25 Intimidator as a direct replacement for the HL-304 series. In support of the class, this carburetor is priced the same as our HL-304WX Dominator, ensuring no additional financial burden for racers.
Now, to address the controversy.
We are generally not in favor of rule changes. However, we have made limited adjustments to better align the performance of Clone and Animal engines, ensuring racers do not feel forced to abandon competitive equipment. In reality, the Clone platform has already displaced the Briggs Animal in many forms of dirt oval racing and is fully capable of sustaining its own Limited Modified class without being combined with other engine platforms.
That said, track and race organizers are often hesitant to eliminate existing classes, and adding new ones can increase logistical complexity. To help ease that transition, we have supported minor rule adjustments to keep the platforms competitive with one another.
The current rule change being proposed by some builders involves increasing the allowable carburetor bore size. The primary motivation is to legalize the use of HL-360 carburetors—originally used on Briggs flathead engines—on Clone platforms. As previously mentioned, the Briggs flathead is no longer supported, and many of its components have become obsolete, often stockpiled by a limited number of builders.
The HL-360D carburetor is not legal in Animal Limited classes and is generally undersized for Open classes. As a result, large quantities of these carburetors remain unused. However, this situation will persist regardless of rule changes, as the entire HL-series has effectively been phased out by Tillotson. From our perspective, the push for this rule change is less about improving class parity and more about creating an outlet for obsolete inventory, rather than adopting carburetors that are purpose-built for the current rules and engine platforms.
We feel obligated to explain why using the HL-360D on an OHV engine is a poor decision. One of the main reasons fewer racers are running Tillotson carburetors today is improper setup—specifically, using carburetors that are not correctly blueprinted for the engine they’re installed on.
Fuel Calibration – Flathead engines require a fundamentally different fuel curve than OHV engines. OHV platforms are significantly more efficient—after all, when was the last time a production automobile used a flathead engine? Because of this, flathead carburetors are much richer on the low-speed circuit. Correcting this for OHV use requires plugging passages beneath the welch plug and redrilling them, which is only one part of properly recalibrating the fuel system.
Additionally, Tillotson diaphragm carburetors are not designed to idle on 4-cycle engines, even though many racers attempt to run them that way. If the low-speed circuit is left in its original flathead configuration, it can create major tuning issues at the track. These include dead spots caused by an overly rich low end, or a lean condition through the midrange and top end. These problems are common and are a key reason why many Open classes avoid diaphragm carburetors altogether. However, when properly set up, a diaphragm carburetor can deliver superior performance.
Throttle Shaft and Linkage – While this is arguably a smaller issue, it is still relevant. The HL-360D and similar carburetors designed for flathead engines pull the throttle shaft from the needle side to avoid exhaust interference. On an OHV Clone engine, this configuration does not align properly. As a result, you will need a linkage conversion kit and, in some cases, a replacement throttle shaft to achieve correct operation. In other words, additional cost and modification are unavoidable.
Intake Manifold – This is another secondary issue, but still important. There are currently no dedicated 1.060” intake manifolds designed specifically for Clone engines. The reason is simple: 1-inch carburetors have historically been the standard for Honda/Clone platforms, dating back to classes like F200, and we have been producing 1-inch Clone carburetors for decades. In contrast, Open Modified engines typically run larger 28mm (1.125”) carburetors.
Taking all of this into account, adapting a 1.060” carburetor to a Clone engine will likely cost $100 or more in additional parts alone. Without a purpose-built intake manifold, you will also need to factor in port matching and full carburetor recalibration, which can add another $150–$200.
In total, you could easily spend $250–$300 modifying a used carburetor just to make it functional in this class. From both a cost and performance standpoint, you are better off purchasing a properly matched HL-304WX—either from an Animal or another Clone setup—than attempting to adapt an HL-360D. At that point, you are also approaching the cost of a brand-new HL-304WX or our all-billet XR-25 Intimidator, both of which are designed specifically for this application.
Now let’s break down the more technical reasons and disadvantages that the HL-360D—or any 1.060” carburetor—has compared to the HL-304WX or our XR-25.
1. Port Velocity and Air Speed
The reason 1.060” carburetors like the HL-360D performed well on flathead engines comes down to airspeed and signal strength. Below is a comparison of stroke and mean piston speed across common platforms:
Engine / Stroke
Piston speed directly influences airspeed through the intake port. Because the flathead has a longer stroke, it generates higher piston speed, which increases air velocity and strengthens the carburetor signal. While increasing port cross-sectional area can increase total airflow (volume), it reduces velocity—which is critical for fuel atomization and throttle response.
When comparing the Clone and Animal, piston speeds are nearly identical. Even at 8000 RPM, the difference is only around 100 ft/min. In contrast, the flathead exceeds the Clone by more than 400 ft/min at 8000 RPM. Even more telling, the flathead at 6000 RPM is within 400 ft/min of the Clone at 8000 RPM. At 4000 RPM—well below racing speeds—the flathead still produces roughly 200 ft/min more than the Clone.
The takeaway is that the flathead generates a stronger airspeed signal to the carburetor, which allows it to tolerate a larger bore with fewer drawbacks.
Looking at it another way, for the Animal to match the flathead’s 8000 RPM piston speed of 3,249 ft/min, it would need to turn approximately 8,849 RPM. The Clone would need to reach roughly 9,170 RPM to achieve the same piston speed.
This highlights the core issue: for a 1.060” carburetor to be truly effective on an OHV Limited engine, it would require significantly higher RPM than these engines are designed to operate within class rules.
While the flathead does not produce the same peak power or torque as OHV platforms, it responds more favorably to larger carburetors due to its higher airspeed. In contrast, the Clone can be negatively affected by reduced port velocity when using an oversized carburetor.
2. CFM and Airflow Demand
The need for higher RPM to match flathead airspeed is only part of the equation. The other limiting factor is airflow (CFM) through the cylinder head.
Class rules inherently restrict airflow, and as RPM increases, the engine’s CFM demand rises accordingly. However, most Clone cylinder heads struggle to exceed 50 CFM at 28” of water at maximum lift. In comparison, carburetors like the HL-360D are capable of flowing nearly double that amount.
If carburetor selection were made without rule constraints, a model like the HL-334WX would be better suited for these engines. It would still exceed the cylinder head’s airflow capacity while maintaining higher intake velocity. This results in a stronger fuel signal, improved atomization, and a broader, more usable power curve.
In contrast, an oversized carburetor reduces velocity without providing a meaningful airflow advantage, since the cylinder head itself becomes the restriction.
3. Tuning Sensitivity and Consistency
As previously mentioned, fuel calibration differs significantly between flathead and OHV engines. However, tuning behavior at the track also varies.
Flathead engines tend to run hotter, and carburetor tuning is often adjusted to manage temperature and prevent overheating. OHV engines, on the other hand, run cooler, yet many racers make the mistake of chasing specific cylinder head temperature targets as a tuning method. This often leads to confusion and inconsistent performance.
These challenges are amplified when the carburetor is not properly matched to the engine. The tuning window becomes narrower as the relationship between the high-speed and low-speed circuits shifts. An engine may perform well on a dyno under controlled conditions, but real-world variables—such as track surface, weather, and race duration—can significantly impact carburetor behavior.
When tuning becomes necessary, an improperly sized or calibrated carburetor can introduce hesitation, dead spots, and inconsistent throttle response—ultimately costing lap time.
We have spent years refining our carburetor designs to minimize these issues. When properly matched and set up, our carburetors typically require little to no adjustment beyond the high-speed needle. In many cases, racers rarely need to make changes at all once properly dialed in.
Our goal extends beyond simply increasing peak power. We focus on improving drivability and tunability—factors that translate directly into faster lap times, greater consistency, and ultimately, better race results.
What’s the Difference Between the HL-304WX Dominator and XR-25 Intimidator?
Since the mid-1980s, EC Birt has been a key developer and importer of Tillotson carburetors in the United States. Carburetors such as the HL-348—and later models like the HL-368 and HL-388, which were based on that platform—were specifically developed by EC for Super Stock, Limited Modified, and F200 classes, all of which utilized 1-inch bore carburetors.
By the late 1990s and early 2000s, the HL-304WX was developed by Carroll Ford, current owner of EC, as a replacement for the HL-348—specifically for OHV engine applications. The HL-304 series proved to be a significant improvement over the earlier designs, offering better overall performance and consistency. Our Dominator versions are arguably among the most refined Tillotson carburetors available for OHV engines, requiring minimal adjustment to achieve optimal performance—allowing many racers to effectively “set it and forget it.”
The XR-25 Intimidator represents the next evolution in that progression. These carburetors were designed to improve upon the original Tillotson architecture in terms of performance, durability, and tunability.
The original X-Series Intimidators were developed as a replacement for the Stage 3+3 “Twilight” carburetors—EC’s highest level of blueprinting and machining applied to Tillotson platforms. Historically, these carburetors ranged from $600 to over $1,000 more than 30 years ago. Even today, 3+3 Twilight versions of the HL-304WX are only available through a select group of builders and are not publicly marketed. The XR-Series builds on that legacy, now produced entirely from billet aluminum.
Manufacturing carburetors from billet provides significantly greater flexibility in design and continuous refinement. Unlike mass-produced cast carburetors—which require large production volumes and slow iteration cycles—billet construction allows for ongoing improvements without those constraints.
Key Improvements of the XR-25 Intimidator
All-Billet Body – As mentioned, the XR-25 features a fully billet-machined body, including the pump plates. The velocity stack is now fully integrated into the carburetor body. Previously, velocity stacks had to be added and carefully machined to blend with the carburetor. This new unibody design improves airflow efficiency while also reducing overall length for better fitment.
All-Billet Pump Plates – The pump plates are fully machined from billet, allowing for improved fuel flow and volume compared to stock cast or modified plates. In general, billet components offer greater precision, consistency, and durability, along with a higher-quality finish.
Throttle Shaft Design – The XR-25 utilizes a slightly larger diameter throttle shaft for increased strength compared to traditional Tillotson designs. Many users have experienced bent or twisted throttle shafts—often due to improper throttle stop setup. To address this, we increased shaft strength while maintaining airflow by thinning the shaft across the butterfly area. Additionally, the revised body design allows for easier repair if damage does occur. Throttle shaft bushings can also be added if needed, further extending service life. The result is a more durable carburetor that is also more serviceable, increasing its long-term value.
Billet Intake Manifold – Changes to the carburetor body required corresponding updates to the intake manifold. As a result, traditional HL-Series intake manifolds are not compatible with the XR-25. While this may initially seem like a drawback, the reality is that the HL-Series platform is being phased out, making a new intake inevitable. Given that this class is still developing—and that the Clone platform represents the future of 4-cycle karting—most racers are already transitioning toward complete, purpose-built carburetor packages.
Proven Performance Components – Carrying over from both the earlier Intimidator Series and our Dominator carburetors are billet steel high- and low-speed needles, along with high-flow red fuel inlet caps. These components have already proven their reliability and performance across multiple platforms.
While the HL-304WX Dominator remains a highly competitive carburetor—and continues to win races—the overall outcome still depends on the complete package: engine builder expertise, driver skill, chassis setup, and tire management.
The XR-25 Intimidator builds on that foundation by incorporating advanced design features and manufacturing techniques typically reserved for much higher-end carburetors. The goal is not just increased performance, but improved consistency, durability, and ease of use.
Perhaps most importantly, the XR-25 delivers these advantages at no additional cost over a standard Dominator carburetor—and at a significantly lower price point than most modern Tillotson offerings.
FAQ
1. What is the best carburetor for a 208cc clone engine?
The best carburetor depends on your class rules and setup, but for most Limited Modified applications, a 1-inch carburetor like the EC Dominator (HL-304WX) or XR-25 Intimidator provides the best balance of airflow, velocity, and tunability.
2. Is a 1.060 carburetor better than a 1-inch carburetor?
Not for most OHV clone engines. While a 1.060” carburetor flows more air, it reduces port velocity and fuel signal, often resulting in worse performance compared to a properly sized 1-inch carburetor.
3. Why do flathead engines use larger carburetors like the HL-360D?
Flathead engines have longer stroke and higher piston speed, which increases air velocity and allows them to utilize larger carburetors more effectively than OHV engines.
4. What is the difference between the EC Dominator and XR-25 Intimidator?
The Dominator is a highly refined cast Tillotson-based carburetor known for consistency and ease of tuning. The XR-25 Intimidator is a fully billet carburetor with improved airflow design, durability, and tunability, offering next-level performance at a similar cost.
5. Do diaphragm carburetors work well on 4-cycle engines?
Yes—when properly calibrated. Diaphragm carburetors can outperform traditional designs, but incorrect setup can lead to tuning issues like hesitation and dead spots.
6. Why is carburetor sizing important in kart racing?
Carburetor size affects air velocity, fuel atomization, and throttle response. Oversized carburetors can reduce performance by weakening the fuel signal and making tuning more difficult.
7. Is the XR-25 Intimidator worth upgrading to?
Yes, especially for racers looking for improved durability, consistency, and performance. Its billet construction and updated design provide advantages over traditional cast carburetors.
8. Can I use an HL-360D on a clone engine?
It is not recommended. The HL-360D requires significant modification, additional cost, and often results in reduced performance compared to carburetors designed specifically for OHV engines.
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